I have completed the "Hardest puzzle in the world" in Lufia II, but I forgot my prizes (there were 4) and I forgot how to get back in. I DO know that the puzzle is in Dragon Mountain, and that a girl greets you and tells you that it's the hardest puzzle in the world.

Once a jolly swag-man camped by a billabong
Under the shade of a Coolabah Tree
And he sang as he watched and he waited until his billy boiled
Who'll come a-waltzing Matilda with me?